This invention relates to synthetic resin pen units in writing devices. More particularly, it relates to a pen unit in the form of a bar which is obtained by fusing a number of synthetic resin elements in the axial direction thereof so that the pen unit comprises a cylindrical member which is annular in section having a number of linear members extending inwardly from the inner wall of the cylindrical member. An improved sectional structure results.
The structure of a synthetic resin pen unit of this type should satisfy the following conditions:
(1) An ink passage has a size large enough to deliver an amount of ink required or consumed in writing.
(2) The ink passage has a small width so that it can introduce the ink in the ink container of the writing device to the pen point and has capillary action to maintain the ink in the pen unit.
(3) The sectional configuration of the ink passage concentrates ink to the center in section of the pen unit.
(4) The pen unit is strong enough to withstand a writing pressure applied thereto.
In order to satisfy the above-described condition (1), a pipe-shaped structure having a circular section ink passage is most effective. However, this structure cannot meet condition (2). In order to satisfy both conditions (1) and (2), it is necessary that the ink passage has a sectional configuration which is small in width and large in length and has a sectional area which is large enough to satisfy condition (1).
Furthermore, the writing device must be such that the user feels comfortable when writing with it. One of the factors which determine whether or not a writing device meets this requirement is the flexibility of the pen unit. Accordingly, the pen unit must be designed so that it has a desired flexibility.
Within the prior art a number of configurations have been proposed for pen units, typified by U.S. Pat. Nos. 3,932,044; 4,072,430 and 4,076,428. The hallmark of each of these prior art configurations is an attempt to maintain the necessary flexibility so that writing comfort is achieved, however, the above four defined conditions are to some extent sacrificed. U.S. Pat. No. 3,932,044 shows best in FIG. 2 of that patent a coreless configuration having curved thin radial slots formed between triangular segments. The triangular segments have different heights and alternate with each other in a symmetrical fashion. They are formed of a number of semi-circles to define a regular triangle and, under writing pressure, the curve segments of the opposite walls tend to engage each other. Under writing pressure, the center conduit or coreless design is maintained.
U.S. Pat. No. 4,072,430 is also directed to a "coreless" design nib shown best in FIG. 3 of that patent. This configuration utilizes an outer wall having a number of cantilevered inner partitions. Each partition is formed of intersecting semi-circles of alternating large and small diameters. As shown in the Figure, each of the inner partitions is formed by having a circle of a large diameter separated by one of a small diameter and additionally, each circle having the same diameter lies along a common circumferential circle about the nib. Circumferential capillary passages are provided in the outer wall.
U.S. Pat. No. 4,076,428 deals with a nib configuration utilizing a number of trunk portions which inwardly project toward the center. Each of the trunk portions have a series of branch elements in particular symmetrical relationships. Various degrees of fold symmetry are defined to achieve the orientation of the tree, stem, and branch portions of each inwardly projecting segment. Core-type configurations are shown in FIGS. 8 and 9 with the hallmark of the patent being the number of inwardly projecting stem portions in the required symmetry about the center.
In each of these prior art pen unit configurations, flexibility at the tip is attained at the expense of the requirements of ink passage delivery and/or structural integrity of the unit under writing pressure. Accordingly, there exists a requirement for an improved synthetic resin pen unit which satisfies the four above listed structural conditions yet attains the necessary flexibility of achieve writing comfort.